Click for next page ( 10

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement

Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 9
9 Segregation and Bleeding Subsidence Because of the high fluidity of CLSM mixtures, the poten- Subsidence occurs when CLSM loses water (through bleed- tial for excessive segregation and bleeding exists, especially ing and absorption into surrounding soil) and entrapped air, with very high water contents. Generally, the use of fly ash and resulting in a reduction in volume. CLSM with high water AEAs is beneficial in minimizing the potential for segregation content has been found to exhibit a subsidence depth equal and excessive bleeding. The use of low-density CLSM with to approximately 1 to 2 percent of the trench depth (McLaren high air contents (e.g., 15 to 35 percent by volume) allows for and Balsamo 1986). The actual amount of subsidence that reductions in water content and bleeding (Hoopes 1998). occurs for a given placement depends on the materials and ASTM C 940, "Expansion and Bleeding of Freshly Mixed mixture proportions used, as well as placement heights, the Grouts for Pre-Placed Aggregate Concrete in the Laboratory," environmental conditions and permeability of surrounding is a simple, but effective method of measuring the total vol- soil. Subsidence generally only occurs during CLSM place- ume and accumulation of bleed water on the top of CLSM. ment and up until the mixture hardens. Using sufficient fines Although there are no commonly used methods available to (e.g., fly ash), accelerating admixtures, or high early-strength measure the segregation of CLSM, visual observations during cement may be effective in limiting subsidence by minimizing mixing and placing serve as good, practical indicators. the propensity for subsidence or decreasing the window of vulnerability of CLSM. Hardening Time Hardening time is the approximate period of time required Hardened CLSM Properties for CLSM to gain sufficient strength to support the weight of Compressive Strength a person (ACI Committee 229 1999). The hardening time can be as short as 1 hour, but generally takes 3 to 5 hours (Smith The compressive strength (or unconfined compressive 1991). The early hardening characteristics of CLSM are af- strength to be consistent with geotechnical terminology) of fected by several parameters, including mixture proportions, CLSM is the most common hardened property measured, climatic conditions, and the surrounding environment, espe- and the one most commonly found in state DOT specifica- cially drainage conditions. Because measuring the early age tions. Compressive strength and flowability were the two most compressive strength of CLSM is not practical, test methods commonly specified CLSM properties in the 1998 survey of for penetration resistance are most commonly used to quan- current practice (Folliard et al. 1999); these and other CLSM tify setting and hardening time. Laboratory penetrometers properties and tests are highlighted in Table 2.2. (e.g., ASTM C 403, "Time of Setting of Concrete Mixtures by CLSM compressive strength values are often used as an Penetration Resistance"), as well as soil pocket penetrome- index for excavatability or digibility (e.g., maximum allowed ters, are commonly used to measure the setting and hard- values of 0.35 to 1.0 MPa), when future excavation may be re- ening of CLSM. Design penetration values are sometimes quired. Materials and mixture proportions must be selected specified to schedule construction practices and the time to to ensure that these strength values are not exceeded in the opening of traffic. Other techniques sometimes used for long term. Also, for some applications, early-age compressive CLSM include the dynamic cone penetrometer and Kelly ball. strength may be specified for constructability reasons (e.g., Table 2.2. CLSM properties typically specified and measured by state DOTs. Number Property of States Common Test Method(s) Testing Flow 18 ASTM D 6103 (or similar) and ASTM C 143 Compressive strength 17 AASHTO T 22 and ASTM D 4832 Unit weight 14 AASHTO T 121 Air content 10 AASHTO T 152 Set time 7 ASTM C 403 Durability 2 pH and resistivity Shrinkage 1 Visual Geotechnical 1 Direct shear Temperature 1 Modified ASTM C 1064 Chlorides/sulfates 1 Determination of ion contents Permeability 0 None Source: Folliard et al. (1999)

OCR for page 9
10 for subsequent paving or opening to traffic). Some applica- actual excavation equipment to assess the ease of excavating tions (e.g., void fill) may not necessarily demand specific CLSM in trenches, and correlations were made with other strength values, and in these cases, strength may not need to CLSM properties, such as unconfined compressive strength be measured. More information on applications of CLSM is (Landwermeyer and Rice 1997). Similar efforts were also part discussed later in this chapter. of the current project, as discussed in Chapters 3 (laboratory The development of CLSM compressive strength is differ- evaluations) and 4 (field studies). ent from conventional concrete in that it is thought to have Many CLSM users have specified maximum unconfined two components of strength: particulate and nonparticulate compressive strength values to ensure that CLSM can be ex- (Bhat and Lovell 1996). The nonparticulate component of cavated at later ages. Another approach, outlined in the Hamil- strength results from the cementitious (and pozzolanic) re- ton County (Ohio) Performance Specification for CLSM, is to action of cement and fly ash with water, whereas the particu- specify a removability modulus, which is both a function of late component of strength is similar in nature to that of 28-day unconfined compressive strength and density of granular soil. Water-cement ratio plays an important role in CLSM in the field. If the calculated value of the removabil- the development of unconfined compressive strength (Bhat ity modulus is less than 1.0, the specific CLSM is considered and Lovell 1996), but in some instances, cement content may to be removable. be more influential (Brewer 1992) or easier to control. The The majority of states require that CLSM compressive type and amount of fly ash (if used) also has a major effect on strengths not exceed some pre-defined early strength in compressive strength, especially on long-term compressive order for the material to meet excavatability requirements. strength. Performance-based specifications based on locally available ASTM D 4832, "Preparation and Testing of Controlled materials in some cases have proven to be acceptable in limit- Low-Strength Material (CLSM) Test Cylinders" is the most ing the long-term strength gain. An alternative approach is to common method used by state DOTs for evaluating CLSM limit the cement content of the CLSM mixes. About 20 per- strength. The most critical potential problem with this and cent of the states place limits on the amount of cement that related compression test methods for CLSM is the relatively can be added to CLSM, thus limiting the ultimate strength of low strength of CLSM. This characteristic low strength cre- the mixture (Folliard et al. 1999). ates difficulties in handling CLSM test specimens (e.g., strip- The ability to predict long-term strength gain is paramount ping cylinders) and in testing cylinders, where large-capacity to assuring that CLSM will remain excavatable. Thus, methods concrete compression machines have poor accuracy in the of predicting strength gain for various combinations of con- required low load range. Many load frames used by research stituent materials were a prime focus on research conducted laboratories for testing CLSM are in the 1,300 to 2,220 kN under this project, and correlations between excavatability and capacity range (Folliard et al. 1999). For a 150 300 mm various CLSM properties (e.g., compressive strength, tensile cylinder with a compressive strength of 1.0 MPa, the maxi- strength, etc.) and test values (e.g., dynamic cone penetrom- mum load at failure is only about 18 kN, or approximately eter) were attempted (see Chapter 3). 1 percent of the load frame capacity. The precision of these larger load frames in the lower compressive load range is not Permeability sufficient in most cases to produce an accurate measure of compressive strength. This problem becomes exacerbated when The permeability of CLSM to both liquids and gases has a smaller diameter cylinders are used or lower strength CLSM significant impact on performance of CLSM in various appli- is used, especially at early ages. Concerns regarding machine cations. The permeability of CLSM affects several important capacity and accuracy, as well as curing conditions, cylinder properties, including drainage characteristics, durability, and mold types, and other aspects of compression testing were leaching potential. An advantage that CLSM has, compared evident in the 1998 survey conducted under this project, and to conventional concrete, is that actual water permeability tests significant emphasis was placed in the laboratory phase of this can be conducted (conventional concrete is too impermeable project (Chapter 3) on improving the test method. A revised for practical measurements of water permeability). version of this test is recommended in Appendix B. The most common method of assessing CLSM permeability is ASTM D 5084, "Measurement of Hydraulic Conductivity of Saturated Porous Materials Using a Flexible Wall Perme- Excavatability ameter." Typical values for CLSM obtained from this test Easy removal of CLSM from trenches is essential when util- method are in the range of 10-4 to 10-5 cm/s, but higher strength ities fail or require repair. Undesired long-term strength gain mixtures may reduce the permeability to as low as 10-7 cm/s may prohibit the removal of CLSM using conventional means (ACI Committee 229 1999). Low-density, air-entrained CLSM of shovels or backhoes. Prior studies have been performed using mixtures tend to have significantly higher permeability. CLSM